Method of preparing surfaces of plastic for electro-deposition

ABSTRACT

THE SPECIFICATION DISCLOSES A METHOD AND COMPOSITIONS FOR PREPARING PLASTIC SURFACES TO RECEIVE A METALLIC DEPOSIT. A SOLUTION COMPRISING A NOBLE METAL SALT AND SULFURIC ACID IS USED TO ACHIEVE SIMULTANEOUS ROUGHENING AND ACTIVATION OF THE SURFACE. IN A PREFERRED EMBODIMENT, THE SOLUTION ALSO CONTAINS AN OXIDIZING AGENT.

United States Patent Office 3,553,085 Patented Jan. 5, 1971 3,553,085 METHOD OF PREPARING SURFACES F PLASTIC FOR ELECTRO-DEPOSITION Kurt Heymann, Berlin, Germany, assignor to Schering AG, Berlin and Bergkamen, Germany No Drawing. Filed Nov. 28, 1967, Ser. No. 686,310 Int. Cl. C23b 5/60 US. Cl. 20430 3 Claims ABSTRACT OF THE DISCLOSURE The specification discloses a method and compositions for preparing plastic surfaces to receive a metallic deposit. A solution comprising a noble metal salt and sulfuric acid is used to achieve simultaneous roughening and activation of the surface. In a preferred embodiment, the solution also contains an oxidizing agent.

The invention relates to the pretreatment of plastic surfaces or the like by chemical roughening thereof with simultaneous activation for the purpose of subsequent electrodeposition.

Plastic surfaces are pretreated by chemical means to roughen the surface with coordinated activation by oxidation.

It is known that the electroplating of plastics requires a pretreatment of the surface to be plated. The processes known heretofore are mainly of a mechanical nature. The injection molded plastic objects are roughened, for example, by tumbling with abrasives, then rinsed, immersed in a solution containing a reducing agent, usually stannous chloride, and are subsequently transferred to an activating solution which contains noble metal salts, mostly silver nitrate or palladium chloride. The portion of the reducing agent adhering from the first bath to the plastic surface reduces the noble metal ions to metal nuclei which stick to the roughened surface by adhesion. Furthermore, it will be noted that such pretreated parts can be metallized further by chemical means. This proc ess requires appreciable time, material and labor, particularly because of the mechanical roughening treatment.

Chemical pretreatment methods, for example using solutions, have also become known in which the plastic surface is roughened or swelled by chemical action. The plastic is activated after such a treatment in the same known manner as in the above-described mechanical roughening process. These chemical processes thus only replace the mechanical roughening of the plastics because it is necessary separately to perform the activating treatment which is necessary for the subsequent electrodeposition.

It has now been found that plastics, particularly of acrylonitrile butadiene-styrene copolymers can be pretreated for electrodeposition in a much more advantageous manner by exposing their surface to a solution which combines chemical roughening or oxidative modification with simultaneous activation. This is achieved by adding to the roughening or oxidizing solution noble metal salts which are capable of penetrating the plastic by diffusion simultaneously with the modification of the surface by the action of this solution. The method of the invention thus combines the roughening and activation in a single step, and is thus much simpler than processes heretofore known. The nuclei formed during the reduction following this pretreatment are anchored relatively deeply in the surface of the plastic. They account for the good adhesion of the metal layers which are applied subsequently by chemical or electrolytic means.

The solutions employed for the method of the invention contain concentrated sulfuric acid and one or more noble metal salts, for example, salts of gold, silver or palladium, such as auric chloride, silver nitrate, silver sulfate, silver acetate, palladium chloride, palladium sulfate and salts of platinum, iridium and osmium, such as the chlorides in preferred amounts averaging from 0.010 to 10 g. of the metal salt per liter of concentrated sulfuric acid. However, the minimum operative concentration of sulfuric acid may be 50% (by weight).

In order to enhance and accelerate the roughening or oxidation of the plastic surface, it is preferred to add oxidizing agents, preferably nitric acid, hydrogen peroxide, chromic acid, or persulfates and the like. Depending on the desired degree of roughening, the method may be performed at temperatures from about 15 to about C. The dwell time of the plastics in the pretreatment bath is variable and is selected from about 30 seconds up to two hours, depending on the amount of electrodeposition to be effected.

The plastic is rinsed after the treatment With the solution of the invention and is transferred to one of the conventional reducing baths. The noble metal ions diffused into the plastic surface are thereby reduced to metal nuclei. The plastic may be metallized chemically at once after this treatment and may be provided thereafter with any kind of metal coating by electrodeposition.

It is not necessary in all instances to reduce the noble metal ions present on the plastic surface in a separate operation, but the pretreated plastics may be metallized directly in a chemical reduction bath. It is only necessary to thoroughly rinse the pretreated plastic and to increase slightly the concentration of the reducing agent in the metallizing bath. Suitable chemical reducing solutions are the copper and nickel baths, for example, which contain formaldehyde or hypophosphite or borohydride as a reducing agent.

Aside from its simplicity, the method of the invention has the advantage that the surface of the plastic parts is much less roughened than in the mechanical pretreating processes. Copper, nickel and chromium may therefore be applied for decorative purposes in much thinner layers for producing a bright appearance. The adhesion of the metal layers to the plastic achieved by this method corresponds to that obtained by mechanical roughening.

The following examples further illustrate, but without limitation, the method of the invention. Parts made of acrylonitrile butadiene-styrene copolymer were treated in the solutions described hereinafter, subsequently rinsed in running water for one minute, and then transferred for one minute to ,a reducing solution consisting of 50 ml. 40% formaldehyde and 50 g. sodium hydroxide per liter, or of 50 g./l. potassium hypophosphite dissolved in 2% to 5% sulfuric acid. The parts so pretreated could be metallized at once by chemical means. The roughening of the plastic surfaces achieved by the solutions described in Examples 3 to 10 was stronger than in the solutions of Examples 1 and 2.

EXAMPLE 1 Composition of the solution: Concentrated sulfuric acid+5 g./1. of silver sulfate.

Temperature: 25 C. Treating time: 3 minutes.

EXAMPLE 2 Composition of the solution: Concentrated sulfuric acid+5 g./l. of silver nitrate.

Temperature: 25 C. Treating time: 3 minutes.

3 EXAMPLE 3 Composition of the solution: Concentrated sulfuric acid+ g./l. of silver acetate+20 ml./l. of concentrated nitric acid.

Temperature: 25 C. Treating time: 3 minutes.

EXAMPLE 4 Composition of the solution: Concentrated sulfuric acid+5 g./l. of silver nitrate+60 g./1. of chromic acid anhydride.

Temperature: 50 C. Treating time: 3 minutes.

EXAMPLE 5 Composition of the solution: Concentrated sulfuric acid+5 g./l. of silver sulfate-+50 ml./l. of 30% hydrogen peroxide.

Temperature: 25 C. Treating time: 3 minutes.

EXAMPLE 6 Composition of the solution: Concentrated sulfuric acid+5 g./l. of silver nitrate+50 g./l. of ammonium persulfate.

Temperature. 25 C. Treating time: 3 minutes.

EXAMPLE 7 Composition of the solution: Concentrated sulfuric aoid+5 g./l. of silver sulfate-{- g./1. of perchloric acid.

Temperature: 25 C. Treating time: 3 minutes.

EXAMPLE 8 Composition of the solution: 70% concentrated sulfuric acid (by weight), 12% phosphoric acid, 0.7% potassium dichromate, 150 mg./l. pd++ in the form of palladium chloride, the balance being water.

Temperature: 65 C. Treating time: from 3 minutes up.

EXAMPLE 9 Composition of the solution: 75% (weight) concentrated sulfuric acid, 12% phosphoric acid, 0.5% potassium dichromate, 100 mg./l. of Au+++ in the form of gold chloride, the balance water.

Temperature: 65 C. Treating time: from 3 minutes up.

EXAMPLE 10 Composition of the solution: 75% (weight) concentrated sulfuric acid, 0.7% potassium dichromate, 100 mg./l. Au+++ in the form of gold chloride, the balance water.

4 Temperature: C. Treating time: from 3 minutes up.

The plastic parts pretreated according to the invention may be metallized in the usual manner, for example, as follows:

The sufficiently rinsed parts are transferred to a bath of the following composition:

Metal deposition is performed at a bath temperature of about 20 to 40 C. for 10 to 20 minutes, whereby copper coatings of about 0.3 to 1.3 microns are deposited depending on the temperature and time of treatment. The deposited meal layer may subsequently be provided with any desired metal coating in the conventional electrolytic baths.

From the foregoing, it will be noted that surfaces made of plastic materials, notably acrylo-nitrile butadiene styrene copolymers can be readily pretreated or roughened superficially by chemical and oxidative means, so that such surfaces will be able to hold thereon metal surfaces by chemical plating and electrodeposition.

What is claimed is:

1. An electrodeposition process consisting essentially of roughening and activating an acrylonitrile-butadiene styrene copolymer plastic surface by contracting said surface with a solution consisting essentially of at least 50% by weight of sulfuric acid, 0.010 to 10 grams of noble metal salt per liter of concentrated sulfuric acid, and an amount of oxidizing agent from the group consisting of nitric acid, hydrogen peroxide and persulfates sufiieient to accelerate roughening and oxidation of said plastic surface; chemically metallizing said roughened, activated sur- I face; and subsequently providing any metal coating thereon by means of electrodeposition.

2. The process of claim 1, wherein the noble metal salt is a salt of a metal selected from the group consisting of gold, silver, palladium, platinum, iridium, and osmium.

3. The process of claim 1, wherein said plastic surface is contacted by said roughening and activating solution at a temperature of from about 15 C. to about C.

References Cited UNITED STATES PATENTS 2,690,402 9/ 1954 Crehan 117-47 3,370,974 2/ 1968 Hepfer 106-1X 3,437,507 4/ 1969 Jensen 106-1X 3,442,683 5/ 1969 Lenoble et al. 117-47 DANIEL E. WYMAN, Primary 'Examiner P. E. KONOPKA, Assistant Examiner US. Cl. X.R. 

